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Nitrogen Distribution in the Pyrolysis Products of a Type II Kerogen (Cenomanian, Italy). Timing of Molecular Nitrogen Production versus Other Gases F. Behar,*,† B. Gillaizeau,†,‡,§ S. Derenne,‡ and C. Largeau‡ Institut Franc¸ ais du Pe´ trole, BP 311, 92506 Rueil-Malmaison Cedex, France, and Laboratoire de Chimie Bioorganique et Organique Physique-UMR CNRS 7573, Ecole Nationale Supe´ rieure de Chimie de Paris, 75231 Paris Cedex 05, France Received July 20, 1999. Revised Manuscript Received October 25, 1999
A type II kerogen (NN8, Cenomanian, Italy) was submitted to isothermal closed pyrolyses, in sealed gold tubes, under various temperature/time conditions (ranging from 280 to 550 °C and from 1 to 72 h). Nitrogen distributions were determined, via independent measurements, in the different pyrolysis fractions. Complete nitrogen mass balances were thus obtained, for the first time to the best of our knowledge, for pyrolyses of a type II kerogen. The nature and the abundance of the individual gases generated under each temperature/time condition were also examined. Taken together the above results provided information on (i) the amounts of N2, CH4, CO2, and H2S produced from the NN8 kerogen upon thermal stress, (ii) the timing of such productions, and (iii) the composition to be expected for the gas fractions generated from such a type II kerogen at different stages of natural maturation. In addition, these observations were compared with those derived from a similar study recently carried out on a type I kerogen.
Introduction Nitrogen is an ubiquitous heteroatom in sedimentary organic matter, although it usually occurs in relatively low abundances when compared to oxygen and sulfur. Nitrogen levels in the 0.5-3% range (wt % of total organic matter) are thus commonly found in low rank coals, oil shales, and immature kerogens,1-8 and still lower values, below 0.1 wt %, are typically observed in most crude oils.5,8,9 Nevertheless, the presence of nitrogen in fossil organic matter and fuels is associated with a number of major drawbacks, including health and environmental hazards via formation of toxic combustion products such as nitrogen oxides,10,11 poisoning of * Corresponding author. † Institut Franc ¸ ais du Pe´trole. ‡ Ecole Nationale Supe ´ rieure de Chimie de Paris. § Present address: CALTECH, 1200 East California Boulevard, Pasadena, CA 91125. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (2) Mu¨ller, E. P.; Goldbecher, K.; Botnewa, T. A. Z. Angew. Geol. 1973, 19, 494-499. (3) Boudou, J. P.; Mariotti, A.; Oudin J. L. Fuel 1984, 63, 15081510. (4) Oh, M. S.; Taylor, R. W.; Coburn, T. T.; Crawford, R. W. Energy Fuels 1988, 2, 100-105. (5) Baxby, M.; Patience, R. L.; Bartle, K. D. J. Petrol. Geol. 1994, 17, 211-230. (6) Krooss, B. M.; Littke, R.; Mu¨ller, B.; Frielingsdorf, J.; Schwochau, K.; Idiz, E. F. Chem. Geol. 1995, 126, 291-318. (7) Littke, R.; Krooss, B.; Idiz, E.; Frielingsdorf, J. AAPG Bull. 1995, 79, 410-430. (8) Barth, T.; Rist, K.; Huseby, B.; Ocampo, R. Org. Geochem. 1996, 24, 889-895. (9) Curiale, J. A.; Bromley, B. W. 17th EAOG Meeting Proc. 1995, 327-328. (10) Schmitter, J. M.; Vajta, Z.; Arpino, P. J. 1980 Adv. Org. Geochem. 1979 1980, 67-76. (11) Dorbon, M.; Schmitter, J. M.; Garrigues, P.; Ignatiadis, I. Org. Geochem. 1984, 7, 111-120.
petroleum-reforming catalysts, and unstability of liquid fuels upon storage as a result of tar formation. Furthermore, numerous accumulations of natural gases worldwide are characterized by contents of molecular nitrogen above 50%, with extremely high values, up to almost 100%, in some cases.6,7,12-15 The existence of such N2-rich natural gases represents a serious exploration risk. Nitrogen origin in these accumulations has thus been extensively discussed but is still a matter of debate.16 Two main types of pathways are currently considered: a “direct ”pathway from N2 of the lithosphere or atmosphere and an “indirect” one via maturation of the nitrogen-containing constituents of sedimentary organic matter. Recent studies showed that the latter pathway is likely to be the major process in the case of the N2-rich gas accumulations occurring in North Germany.6,7,14,17 A relatively large number of laboratory pyrolyses were therefore performed, especially in the past few years, to examine the fate of nitrogen functions upon a thermal stress.4,6-8,14,17-24 These studies were mostly carried out with coals but some kerogens, oil shales, and recent samples were also examined. Nevertheless, the amount of N2 evolved was determined only in a limited number of cases,6,7,14,17,20,23,24 and the general problem of the fate (12) Jenden, P. D.; Kaplan, I. R.; Poreda, R. J.; Craig, H. Geochim. Cosmochim. Acta 1988, 52, 851-861. (13) Whiticar, M. J. Adv. Org. Geochem. 1989 1990, 531-547. (14) Krooss, B. M.; Leythaeuser, D.; Lillack, H. Erdo¨ l Kohle-ErdgasPetrochem. 1993, 46, 271-276. (15) Fang, H.; Sitian, L.; Yongchuan, S.; Qiming, Z. Org. Geochem. 1996, 24, 363-375. (16) Gerling, P.; Idiz, E.; Everlien, G.; Sohns, E. Geol. Jahrb. 1996, D103, 65-85. (17) Idiz, E.; Kross, B. M.; Horsfield, B.; Littke, R.; Mu¨ller, B. 17th EAOG Meeting Proc. 1995, 1089-1091.
10.1021/ef990157g CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
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Table 1. Elemental Composition (wt %), Rock-Eval Analysis, H/C and O/C Atomic Ratios of NN8 Kerogena C
H
O
N
Stot
Sorga
Fe
S2 (mg HC/g)
HI (mg HC/g C)
H/C
O/C
57.95
4.86
7.84
2.14
14.34
2.51
10.35
273
471
1.01
0.10
a
The organic sulfur content of the kerogen (Sorg) was calculated from Stot and Fe abundance via the usual way, i.e., by assuming that iron exclusively occurs as pyrite (FeS2).
of nitrogen functions during sedimentary organic matter maturation is still far from being solved. In fact, to the best of our knowledge, complete nitrogen mass balance has been established, so far, in only one case,24 via laboratory maturation in sealed tubes of a type I kerogen from a large Oligocene lacustrine deposit (Go¨ynu¨k oil shale, Turkey). It is well documented that marine type II kerogens are much more abundant, on a global scale, than lacustrine type I deposits.1 Accordingly, the present work was focused on the examination of a type II kerogen (Cenomanian, Umbria-Marche Appenines, Central Italy) which exhibits a substantial nitrogen content. This kerogen was submitted to isothermal, closed pyrolyses in sealed gold tubes with an inert atmosphere, under various time/temperature conditions. Such a method was previously shown25-28 to provide suitable conditions for simulating the natural maturation of kerogens upon burial. The main purposes of the study were (i) to establish bulk mass balances for nitrogen, along with nitrogen distributions (gases, soluble pyrolysis products, insoluble residues), via examination of the fractions obtained under the above conditions, (ii) to determine the nature and the abundance of the nitrogencontaining gases generated, (iii) to compare the timing of their production with the production of other gases, and (iv) to compare these results from a type II kerogen with those previously obtained, under the same experimental conditions,24 with the type I kerogen of Go¨ynu¨k. Experimental Section Sample Origin. The studied material was collected from a black level (termed NN8) 12 m below the Cenomanian/ Turonian boundary, in a recently exposed section located at Santa Maria de Burano.29 The material was sampled from the organic-rich clayey part of the level (total organic carbon, TOC ) 13.6%). Such organic-rich black shales span a large area in the Umbria-Marche Appenines of Central Italy and a number, including NN8, were correlated over the entire Basin.30 In fact, similar organic-rich black shales occur in many locations, throughout the world, at or near the Cenomanian/Turonian boundary and their occurrence is considered to reflect oceanic anoxic events31 that promoted organic matter accumulation in marine sediments. Kerogen concentrate was prepared from the ground, bitumen-free rock via the classical HF/HCl treat(18) Sinninghe Damste´, J. S.; Eglinton, T.; De Leeuw, J. W. Geochim. Cosmochim. Acta 1992, 56, 1743-1751. (19) Gelin, F.; Boussafir, M.; Derenne, S.; Largeau, C.; Bertrand, Ph. Lecture Notes Earth Sci. 1995, 57, 31-47. (20) Klein, J.; Ju¨ntgen, J. Adv. Org. Geochem. 1971, 647-656. (21) Harrison, W. E. Chem. Geol. 1978, 21, 315-334. (22) Rohrback, B. G.; Peters, K. E.; Sweeney, R. E.; Kaplan, I. R. Adv. Org. Geochem. 1981, 819-823. (23) Boudou, J. P.; Espitalie´, J. Chem. Geol. 1995, 126, 319-333. (24) Gillaizeau, B.; Behar, F.; Derenne, S.; Largeau, C. Energy Fuels 1997, 11, 1237-1249. (25) Monthioux, M. The`se d’e´tat, Universite´ d’Orle´ans, 1986, 331p. (26) Horsfield, B.; Disko, V.; Leistner, F. Geol. Rundsch. 1989, 78/ 1, 361-374. (27) Behar, F.; Kressmann, S.; Rudkiewicz, J. L.; Vandenbroucke, M. Org. Geochem. 1992, 19, 173-189. (28) Vandenbroucke, M.; Behar, F.; San Torcuato, A.; Rullko¨tter, J. Org. Geochem. 1993, 20, 961-972.
ment.32 Pyrolysis experiments were carried out after elimination of the bulk of the mineral matrix so as to avoid artifacts related to matrix effects: water is released from clay minerals upon laboratory pyrolyses leading to a strong increase in catalytic activity, but this is not the case under natural conditions (kerogen cracking then occurs at temperatures below 150 °C when water is still present in substantial amounts and this catalytic effect of clay minerals is at least highly reduced). The bulk features of the concentrate, derived from RockEval pyrolysis and elemental analysis, are reported in Table 1. Some minerals, including pyrite, are known to survive HF/ HCl attacks, and residual ash in the isolated concentrate almost exclusively coresponds to pyrite. Based on its bulk features, the NN8 kerogen appears as a type II kerogen with a relatively high nitrogen content (above 2 wt %) and oil potential (HI ) 471 mg HC/g org C), located at the beginning of the oil window. The Sorg content is relatively low when compared to the average values observed in type II kerogens.33 Pyrolyses. Closed pyrolyses34,35 were carried out under isothermal conditions at temperatures between 280 and 550 °C during time intervals ranging from 1 to 72 h. All these experiments were performed in sealed gold tube (60 mm length, 9 mm i.d., and 0.5 mm thick) as previously described.34 The tubes were welded at one end and weighed before and after sample loading (typical sample size of 30 to 150 mg). The loaded tubes were flushed with argon several times to remove air, then welded at the second end under argon. The tubes were placed in cells in a preheated furnace, so that the heatup time of the sample was around 10 min and pyrolysis duration was measured from the time when the isothermal temperature was reached. The cells were kept with argon at a pressure of 120 bar, during the entire course of the experiment, so as to avoid bursting of the gold tubes. At the end of the heating time, the cells were removed from the furnace, rapidly cooled with compressed air to a temperature of 150 °C, and then immersed in a water bath at room temperature (cooling time < 2 min). For each experiment, two tubes were heated and the pyrolysis products were fractionated according to their volatility and solubility,24,34,36 as described below. Gas Analysis. The first gold tube was pierced with a stainless steel needle in a vacuum line (at 10-5 MPa) connected to a variable temperature trap. Permanent gases (like H2, N2, CO, CH4, and Ar) were volatilized into the line and the condensable gases were trapped at -196 °C. The permanent gases were concentrated by a Toepler pump into a calibrated volume in order to quantify their total amount and finally recovered in a transfer vial for subsequent molecular analysis by gas chromatography (GC). Thereafter, the variable temperature trap was heated to -90 °C, allowing the total amount (29) Salmon, V.; Derenne, S.; Largeau, C.; Beaudoin, B.; Bardoux, G.; Mariotti, A. Org. Geochem. 1997, 27, 423-438. (30) Beaudoin, B.; Mban, E. P.; Montanari, A.; Pinault, M. C. R. Acad. Sci. Paris 1996, 323/IIa, 689-696. (31) Schlanger, S. O.; Jenkyns, H. C. Geol. Mijnb. 1976, 55, 179184. (32) Durand, B.; Nicaise, G. In Kerogen; Durand, B., Ed.; Edition Technip: Paris, 1980. (33) Tomic, J.; Behar, F.; Vandenbroucke, M.; Tang, Y. Org. Geochem. 1995, 23, 647-660. (34) Behar, F.; Leblond, C.; Saint-Paul, C. Rev. I. F. P. 1989, 44, 387-411. (35) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985, 8, 275-292. (36) Behar, F.; Ungerer, P.; Kressmann, S.; Rudkiewicz, J. L. Rev. I. F. P. 1991, 46, 151-181.
Nitrogen in the Pyrolysis Products of a Type II Kerogen of condensable gases (like CO2, H2S, and C2-C5 alkanes) to be quantified by the same procedure as above. These compounds were recovered in the same transfer vial as the permanent gases for analysis. Molecular characterization and quantification (absolute amount and relative abundance of individual compounds) of the “gas” fraction thus recovered were performed by gas chromatography.34 The detectors were calibrated with a standard gas mixture provided by Air Liquide. “C6+” Extract. For the recovery of this fraction, the second tube was immersed into dichloromethane (DCM) and pierced with a stainless steel needle under atmospheric pressure. The tube was then cut into small pieces. So as to ensure an efficient extraction, the bulk of the solid residue was then transferred, with DCM, in an agate mortar and thoroughly crushed for 5 min. The suspension thus obtained and also the gold pieces, which a part of the residue remained tightly associated with, were then combined and refluxed in DCM for 1 h. Afterward the DCM extract was filtered (0.45 µm HA type filter) into a tared flask (filtration was performed at room temperature, after cooling of the extract, so that loss of volatile components during filtration was negligible). This extract contained both relatively volatile compounds, such as the C6-C13 n-alkanes, which are lost upon DCM evaporation and compounds with higher molecular weights which are retained but could comprise non GC-amenable constituents. For quantification this extract was therefore accurately divided in two aliquots. The first one was injected as such, i.e., without solvent evaporation into an on-column GC equipped with an autosampler and a flame ionization detector (FID). Internal and external standards were used. From this analysis, the absolute quantification of the “C7C13” fraction was carried out by integrating the total area of the chromatogram, taking into account the hump and all the GC peaks up to and including the C13 n-alkane, and subtracting the blank (the abundance of the C6 n-alkane could not be determined since it coelutes with the solvent). For the quantification of the “C13+” fraction, the second part of the DCM extract was evaporated under nitrogen and weighed. The absolute amount of the total “C6+” extract thus corresponds to the sum of the “C7-C13” fraction evaluated by GC and of the directly weighed “C13+” fraction. The insoluble residue obtained after the DCM extraction was dried by heating at 50 °C for a few hours and then under vacuum at room temperature. The “C13+” fraction and this dried residue were stored under argon atmosphere before being submitted to elemental analysis. Elemental Analysis. C and H contents were determined by thermal conductivity measurements while coulometry was used for O and S contents. Independent measurements of nitrogen content were obtained via thermal conductivity, using a specific apparatus (Nitromatic 500) with a detection limit of 500 ppm. Iron content was determined by atomic absorption spectrometry. Insoluble Residue. Precise measurement of the absolute amount of the insoluble organic residue cannot be obtained by direct weighing since some losses occur during processing (mostly due to tight association of significant amounts of residue on tube wall). Hence, the amount of insoluble organic matter retained after the pyrolysis experiments was calculated from the carbon mass balance as previously described.24 In brief, this value was simply obtained as the ratio of the absolute amount of carbon in the total organic residue (g of carbon) and of its carbon content (g of carbon/g of residue). The latter value was directly obtained by elemental analysis of the recovered residue. The former was calculated by difference between the unheated kerogen and the sum of all the other fractions obtained by its pyrolysis, by using the results obtained by (i) the elemental analysis of this kerogen, (ii) the GC analysis of the “gas” fraction, and (iii) weight measurement and elemental analysis of the “C13+” fraction
Energy & Fuels, Vol. 14, No. 2, 2000 433 (elemental analysis could not be performed on the “C7-C13” fraction, therefore it was assumed that its C content is similar to the one of the “C13+” fraction). Water. Substantial amounts of water are formed upon pyrolysis of NN8 kerogen but they could not be directly measured under our experimental conditions. The absolute amount of water was therefore calculated as above for the insoluble organic residue but using, now, the oxygen balance. Hence, this value was obtained as the ratio of the absolute amount of oxygen occurring in the water formed upon pyrolysis (grams of oxygen, calculated from the oxygen mass balance) and the oxygen content of water (g of oxygen/g of water). MH2O was therefore calculated from the results of weight measurements, elemental analyses, and from GC analyses. The two calculations, concerned with the mass of insoluble organic residue and the mass of water, are thus entirely independent since they rely on carbon and oxygen mass balances, respectively. Nitrogen Mass Balances. To obtain reliable nitrogen mass balances, any source of contamination by atmospheric N2, which may occur during the analysis of the “gas” fractions, must be identified and reduced as much as possible. In a previous study37 we showed that such a contamination can take place at three stages: when welding the tubes, when recovering gases in the vacuum line, and during GC analysis. A new experimental procedure, allowing us to strongly reduce and to control the contamination by atmospheric nitrogen, was also established.38 This procedure was used in the present study and was tested by running control experiments with empty tubes. The tubes were pierced in the vacuum line, and the released gases were quantified and analyzed by GC as described above. Only very low levels of contamination were thus observed, when simulating gas analysis, with an average value around 3.0 µmol. Accordingly, this amount was systematically subtracted from all the values obtained, for N2, when analyzing by GC the “gas” fractions generated upon pyrolysis of the kerogen. The nitrogen content of the “gas” fraction was obtained from GC analysis; the contents of the unheated kerogen, the “C13+” extract, and the insoluble residue were determined by elemental analysis (elemental analysis could not be performed on the “C7-C13” fraction, therefore it was assumed that its N content is similar to the one of the “C13+” fraction). The measurements derived from duplicate elemental analyses showed only slight differences. A maximum and a minimum nitrogen content could thus be calculated for the initial kerogen, the extract, and the residue. The absolute amounts of nitrogen in the above materials were then calculated from their mass and from these elemental analyses. For each pyrolysis, two nitrogen mass balances were thus determined by comparing (i) the maximal amount of nitrogen in the initial material with the sum of the minimal amount of nitrogen in the different pyrolysis fractions (“gas”, “C6+”, insoluble residue) and vice versa. As indicated in the Results and Discussion section, close values were nevertheless obtained from these “extreme-case” calculations, thus indicating that precise measurements (through weighing, elemental and GC analyses) were achieved.
Results and Discussion Closed pyrolyses of NN8 kerogen were carried out under six temperature/time conditions so as to examine (i) the fate of the nitrogen occurring in this type II kerogen when submitted to an increasing thermal stress and its distribution among the different fractions thus generated, and (ii) the timing of N2 production versus (37) Latourte, L. Rapport de DEA de Chimie Analytique - INSTN, 1995. (38) Gillaizeau, B.; Behar, F.; Derenne, S.; Largeau, C. In preparation.
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Table 2. Mass (wt % of the initial kerogen) of the Different Fractions Obtained by Closed Pyrolysis of NN8 under Various Temperature/Time Conditions and Mass Balances “gas” T (°C) t (h) HC non HC H2O “C6+” wt lossa resb balancec 280 300 350 350 350 375
1 1 1 24 72 72
ndd ndd 0.4 2.1 3.6 10.9
1.3 1.6 2.4 3.5 4.6 7.8
0.1 0.8 2.1 2.3 2.4 2.4
2.5 3.0 12.7 17.6 22.1 10.3
3.9 5.4 17.6 25.5 32.7 31.4
94.5 93.6 81.5 73.8 66.4 69.3
98.4 99.0 99.1 99.3 99.1 100.7
a wt loss corresponds to the sum of all the products released upon pyrolysis (“gas”, H2O, and “C6+”). b res is the insoluble organic residue. The variations in the abundance of the insoluble residue obtained from this type II kerogen are similar to those previously observed, with the same pyrolysis method, for type I, II, and II-S kerogens,24,33,36 i.e., some increase at high severities. On the contrary, a continuous decrease in the amount of insoluble residue was noted, with increasing severity, for type III kerogens and coals.55,57 c Summed mass of all the organic products recovered after pyrolysis (wt % of the initial sample). d nd: not detected.
other gases. To this end reliable information was required, at first, on the mass of the fractions (“gas”, “C6+”, insoluble residue) generated upon pyrolysis and on the production of the other gases. Bulk Mass Balances. As mentioned in the Experimental section, the mass of the different fractions generated upon closed pyrolyses (Table 2) was derived from calculations based on direct weight measurements, elemental analyses (Table 3), and GC analyses (Table 4). In all the tables, the results are ordered according to increasing temperature and time. Absolute values were determined, in each experiment, for the mass of the various fractions. However, for the sake of simplicity, the values in Table 2 are reported as percent of the initial mass of the unheated kerogen, so as to make possible direct comparisons between the experiments under different temperature/time conditions. The summed mass of the pyrolysis fractions is always close (98.4-100.7%) to the mass of the starting organic material. This very good agreement confirms that precise and reliable measurements were obtained for the mass of all these different fractions. When the results obtained under the six temperature/ time conditions are compared (Tables 2 and 4), two phases can be distinguished: • From 280 °C/1 h up to 350 °C/72 h, the amount of insoluble residue markedly decreases whereas the yields of the “C6+” and “gas” fractions increase. These conditions therefore correspond to the main phase of cracking of the kerogen and such a primary cracking is almost completed after 72 h at 350 °C. Indeed, the atomic H/C and O/C ratios of the insoluble residue then obtained (0.64 and 0.04, respectively) are in agreement with the values commonly observed, for the end of the so-called “oil window”, when type II kerogens are plotted in a Van Krevelen diagram.1 This is also confirmed by the comparison of the total amount of light hydrocarbons and “C6+” compounds, 257 mg/g, generated under the latter conditions (Table 2) and of the value of the S2 peak from Rock-Eval pyrolysis (273 mg HC/g). The ratio of these two values indicates that the bulk of the total potential of the NN8 kerogen (ca. 94%) is released at 350 °C/72 h.
• Between 350 °C/72 h and 375 °C/72 h, the “gas” production exhibits a large increase, especially for the hydrocarbon components. Moreover, the amount of insoluble residue significantly increases whereas the yields of the “C6+” fraction sharply decreases. This lowering of the abundance of the “C6+” extract, along with a pronounced increase in the production of methane and C2-C5 hydrocarbons (Table 4), reflects the occurrence of secondary cracking reactions. Accordingly, the production of light hydrocarbons shall then mainly result from the cracking of “C6+” compounds. However, such a production does not account, alone, for the total decrease of the “C6+” fraction. In fact, the increasing amount of the insoluble residue mentioned above shows that the transformation of the “C6+” compounds into insoluble material also becomes important. Therefore, the decrease in the abundance of the “C6+” fraction probably reflects both secondary cracking reactions producing light hydrocarbons and aromatization reactions yielding additional insoluble organic residue, as previously observed for closed pyrolyses of type II kerogen from the Toarcian Shale of the Paris Basin40 and of crude oils.41 The occurrence of the two above phases is also supported by variations in n-alkane distribution in the “C6+” fractions (Figure 1). No large change is observed during the main phase of kerogen cracking and the n-C15/n-C25 ratio only exhibits slight increases from ca. 4.6 to 5 (Figure 1a-c). These weak variations reflect the onset of secondary cracking and it was previously observed, under the same conditions, that only 0.3, 6.2, and 17.5% of the C25 n-alkane, respectively, are then converted into lighter hydrocarbons by secondary reactions.42 In contrast, this conversion yield is much higher, ca. 75%, at 375 °C/72 h.42 Indeed, for the latter conditions, a n-C15/n-C25 ratio of 7.1 is observed (Figure 1d) and reflects a large level of secondary cracking. Study of Non-Nitrogen Gases. The composition of the “gas” fractions obtained for the different temperature/time conditions is reported in Table 4. As commonly observed in closed pyrolyses,43,44 molecular hydrogen is a relatively minor product although its abundance regularly increases with severity. Methane and C2-C5 hydrocarbons are minor pyrolysis products up to 300 °C/1 h. Afterward their abundance markedly increases along with severity (Table 2). Thus, the total mass of these light hydrocarbons corresponds to ca. 10 wt % of the initial organic matter for pyrolysis at 375 °C/72 h. However, as already discussed, the secondary cracking of the “C6+” compounds is then probably the main source of such hydrocarbons and only small amounts of light hydrocarbons are generated via primary cracking. Such features are consistent with the occurrence of most CH2 groups in long hydrocarbon chains, for NN8 kerogen, in agreement with (i) GC (39) Madec, M.; Espitalie´, J. J. Anal. Appl. Pyr. 1985, 8, 801-819. (40) Behar, F.; Vandenbroucke, M.; Tang, Y.; Marquis, F.; Espitalie´, J. Org. Geochem. 1997, 26, 321-339. (41) Schenck, H. J.; Di Primio, R.; Horsfield, B. Org. Geochem. 1997, 26 467-481. (42) Behar, F.; Vandenbroucke, M. Energy Fuels 1996, 10, 932940. (43) Higgs, M. D. In Habitat of Palaeozoic Gas in N. W. Europe; Broocks, J., Goff, J. C., van Hoorn, B., Eds.; Geological Society of London, 1986; pp 113-120. (44) Behar, F.; Vandenbroucke, M.; Teermann, S. C.; Hatcher, P. G.; Leblond, C.; Lerat, O. Chem. Geol. 1995, 126, 247-260.
Nitrogen in the Pyrolysis Products of a Type II Kerogen
Energy & Fuels, Vol. 14, No. 2, 2000 435
Table 3. Elemental Composition (wt %) of the “C6+” Fractions and the Insoluble Residues Obtained by Closed Pyrolysis of NN8 Kerogen under Various Temperature/Time Conditions, Mass Balances, and Atomic Ratiosa “C6+” fraction T (°C)
t (h)
C
H
Ob
280 300 350 350 350 375
1 1 1 24 72 72
75.15 76.60 80.88 84.70 85.54 82.45
7.50 8.13 8.58 8.79 9.09 8.11
13.06 11.00 6.52 3.25 2.77 3.49
insoluble residue
N
Sorg
balancec
C
H
Ob
N
Stotd
Fe
balancec
H//C
O//C
2.99-3.18 3.01-3.03 2.77-2.84 2.25-2.38 2.04-2.04 1.66-1.74
0.52 0.70 0.92 0.90 1.04 4.21
99.32 99.45 99.71 99.96 100.48 99.96
59.44 59.30 57.90 55.54 53.40 56.72
4.97 4.88 4.10 3.64 2.86 2.34
6.86 6.06 4.33 3.87 3.06 1.83
2.27-2.32 2.23-2.30 2.15-2.24 2.32-2.34 2.42-2.48 2.53-2.58
13.83 14.27 16.31 17.83 19.28 18.30
11.15 11.10 13.00 14.50 15.95 15.45
98.10 98.88 97.84 97.71 97.00 97.20
1.00 0.99 0.85 0.79 0.64 0.50
0.09 0.08 0.06 0.05 0.04 0.02
a These values were calculated from both GC analyses and elemental analyses as discussed in the experimental section. b Oxygen was directly measured. c Summed abundance of all the analyzed elements as wt % of the total fraction considered (the average value from N was used for these calculations). The summed values thus obtained are always very close to 100%, reflecting a high accuracy for the measurements of the different elements. d A precise calculation of Sorg from Stot and Fe abundance (see footnote a, Table 1) is difficult for these insoluble residues since, after heating under such conditions, pyrite is partly transformed into pyrotite (FeS) and Fe2S3.
Table 4. Distribution of the Gaseous Compounds (µmol/g of initial kerogen) Resulting from Closed Pyrolysis of NN8 Kerogen for Various Temperature/Time Conditionsa T (°C) 280 300 350 350 350 375 550
t (h) 1 1 1 24 72 72 24
H2
CO
ndf
ndf
ndf 1.8 40.0 64.4 178.3 37.4
ndf ndf ndf ndf ndf 7.4
CO2 277.1 358.7 539.7 759.0 957.3 1328.6 200.6
N2b () () () 20.1 28.7 93.6 ()
H2S 2.6 4.8 14.8 42.1 82.2 475.7d 247.3e
C1 7.4 18.3 107.6 423.7 682.0 2262.9 988.2
C2 0.9 2.7 31.6 188.5 340.5 1003.9 ndf,g
C3
iC4c
nC4c
C5
0.5 1.1 15.3 104.6 177.8 533.0 ndf,g
ndf
ndf ndf 3.4 31.5 66.7 188.9 ndf,g
ndf ndf 0.9 15.1 32.3 64.2 ndf,g
ndf 2.1 11.2 20.0 63.6 ndf,g
a All the constituents in the gas fraction were identified by comparison with references (including, in addition to N , various N-containing 2 gas: NO, NO2, and NH3). The 550 °C experiments correspond to further heating of the insoluble residues from the 375 °C/72 h pyrolysis. Accordingly, these results cannot be precisely compared with the other figures of the table since the latter were derived from direct pyrolyses at the considered temperature/time conditions. b The absolute amounts used for calculating the production of N2 from 1 g of kerogen were obtained, as indicated in the Experimental Section, by subtracting 3 µmol (i.e., the residual contamination determined via the control experiments) from the measured values. The corrected value thus obtained was only 1 µmol in the case of the 280 °C/1 h, 300 °C/1 h, and 350 °C/1 h experiments thus reflecting a very weak, close to detection limit and not significant, N2 production (). c The iC4/nC4 ratio is sometimes used as a maturity index. In the present study, for NN8 pyrolyses, no regular trend was noted for higher severities; however, this ratio is always 20%) is still retained, after 72 h at 375 °C, in the insoluble material. Previous studies on the fate of sulfur in fossil organic matter, upon a thermal stress, were chiefly concerned with coals,46-50 and coal pyrolysis in these experiments was exclusively performed with open systems. It thus appeared that aliphatic sulfides (the main sulfurcontaining groups in low rank coals) are cleaved at relatively low temperatures. The thermal degradation of such groups results in H2S evolution or in conversion into aromatic sulfur forms. Furthermore, recent studies on closed pyrolyses of brown coals51 showed that H2S can also react with the residual coal to produce new organic sulfur structures with a high thermal stability. Sulfur-containing aromatic groups in coals (native or generated upon maturation) exhibit a much higher thermal stability than aliphatic sulfides and they can (46) Kelemen, S. R.; Vaughn, S. N.; Gorbaty, M. L.; Kwiatek, P. J. Fuel 1993, 72, 645-653. (47) Calkins, W. H. Energy Fuels 1987, 1, 59-64. (48) Torres-Ordonez, R. J.; Calkins, W. H.; Klein, M. T. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429, Washington, DC, 1990; pp 287-295. (49) Boudou J. P. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429, Washington, DC, 1990; pp 345-364. (50) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J.; Sansone, M. Fuel 1991, 70, 396-402. (51) Zhiguang, S.; Batts, B. D.; Smith, J. W. Org. Geochem. 1998, 29, 1469-1485.
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Figure 1. GC traces of the C6+ fraction during artificial maturation of the NN8 kerogens at various T/t conditions in closed pyrolysis system. Table 5. Sulfur Distribution (wt % of the total amount of S in the initial kerogen) in the Different Fractions Obtained by Closed Pyrolysis of NN8 Kerogen under Various Temperature/Time Conditions and Sulfur Balances T (°C)
t (h)
“gas”a
“C6+”
resb
280 300 350 350 350 375
1 1 1 24 72 72
0.3 0.6 1.9 5.4 10.5 60.7c
0.5 0.8 4.7 6.3 9.2 17.3
99.2 98.6 93.4 88.3 80.3 22.0
a H S was the only S-containing gas generated during pyrolyses. 2 res is the insoluble organic residue. As already stressed (footnote d, Table 3), a precise calculation of Sorg from Stot and Fe measurements is difficult for these pyrolysis residues due to partial degradation of pyrite upon heating; accordingly Sorg was determined by difference. c Overestimated value (see footnote d, Table 4); the percentages corresponding to the “C6+” fraction and the insoluble residue are therefore underestimated.
b
produce H2S only at a high temperature. The very weak production of H2S observed from NN8, under low severities, points to a low contribution of aliphatic sulfides to total sulfur in this kerogen and, hence, to a major contribution of sulfur-containing aromatic groups. The substantial amount of sulfur retained into the insoluble residue, after pyrolysis at 375 °C for 72 h (Table 5), should reflect the presence of aromatic, sulfurcontaining groups with a high thermal stability. These groups are probably included in polyaromatic structures formed via the polycondensation-aromatization processes that are important, as already discussed, under such temperature/time conditions. Only a few quantitative studies have been concerned with sulfur fate during kerogen pyrolyses so far.24,33,52 Two of these studies24,33 were also carried out in sealed gold tubes under various temperature/time conditions. (52) Baskin, D. K.; Peters, K. E. AAPG Bulletin 1992, 76, 1-13.
The former was performed on a type I kerogen (Go¨ynu¨k), whereas the latter was carried out on a classical type II-S kerogen from the Monterey Formation with an organic sulfur content of ca. 11 wt %. Comparisons with the present results, obtained via the same pyrolysis method, revealed some conspicuous differences concerned with sulfur fate. Thus, for the two kerogens previously studied, substantial amounts of H2S were generated under low severities and about 50% of the total sulfur is released as H2S during the 350 °C/24 h experiments. In contrast, only weak productions of H2S are observed from NN8 under low severities and, at 350 °C/24 h, this gas accounts for only ca. 5% of the total sulfur of the initial kerogen. As mentioned above, the extent of the early production of H2S from fossil materials is likely related to the abundance of aliphatic sulfides. Accordingly, the observed difference should reflect, at least in part, differences in maturity since the two kerogens previously studied are immature whereas NN8 underwent some maturation. In fact, as shown by its bulk features, including a relatively low H/C atomic ratio of ca. 1 instead of 1.27 for the standard, immature, type II kerogen from the Toarcian Shale of the Paris Basin,27 this kerogen is located at the beginning of the oil window. The limited maturation of NN8 would result in a low contribution of aliphatic sulfides while such sulfur groups would be more abundant in the other two samples, as recently shown by temperature-programmed reduction, indicating that a large part of total sulfur in the Go¨ynu¨k kerogen should occur under aliphatic forms.53 Another difference is concerned with sulfur distribution in the soluble fraction. Thus, a significant part of the total sulfur is found in the “C6+” compounds in the case of the Monterey and NN8 (53) Mitchell, S. C.; Snape, C. E.; Garcia, R.; Ismail, K.; Bartle, K. D. Fuel 1994, 73, 1159-1166.
Nitrogen in the Pyrolysis Products of a Type II Kerogen Table 6. Nitrogen Distribution (wt % of the total amount of N in the initial kerogen) in the Different Fractions Obtained by Closed Pyrolysis of NN8 Kerogen under Various Temperature/Time Conditions and Nitrogen Balances T (°C) 280 300 350 350 350 375
t (h) 1 1 1 24 72 72
“gas”a e e e 2.6 3.7 12.2
“C6+”b
resb,c
balanced
3.5 ((0.1) 4.1 ((0.0) 16.2 ((0.2) 18.6 ((0.5) 20.5 ((0.1) 8.0 ((0.2)
98.8 ((1.2) 96.5 ((1.6) 81.5 ((1.7) 78.3 ((0.5) 74.1 ((1.1) 80.7 ((0.9)
102.3 ((1.3) 100.6 ((1.6) 97.7 ((1.9) 99.5 ((1.0) 98.3 ((1.2) 100.9 ((1.1)
a N was the only N-containing gas generated during pyrolyses. 2 Values calculated, as indicated in the experimental, from the minimum and maximum N contents derived from duplicated elemental analyses. c res corresponds to the insoluble organic residue. d Summed abundance of the nitrogen recovered in the different fractions (wt % of total N in the initial sample). e : see footnote b, Table 4.
b
samples whereas only negligible amounts occur in the soluble products generated from the type I Go¨ynu¨k kerogen. Nitrogen Distribution in Pyrolysis Products and Nitrogen Balances. The amount of nitrogen in the pyrolysis fractions (“gas”, “C6+”, insoluble residue), expressed as % of the total amount of N in the initial kerogen, is reported in Table 6. As discussed in the Experimental Section, the value for each fraction was independently determined. The summed amounts of the nitrogen recovered in these different fractions, for the six temperature/time conditions tested, account for ca. 98 to 102% of the total nitrogen occurring in the starting material. Precise balances are thus obtained for nitrogen, despite the large number of measurements required for establishing such balances, thus reflecting the precision and reliability of these measurements. It appears that the bulk of the nitrogen (ca. 74 to 99%) is always retained in the insoluble residue. As thermal stress increases, the nitrogen in this residue decreases to a minimum for the 350 °C/72 h experiment and then substantially increases. Reverse variations are noted for the % of total nitrogen in the “C6+” fraction. The increase in the % of nitrogen in the insoluble residue, observed for the highest severity, thus partly compensates the decrease in the latter fraction. As already discussed in the previous section, condensation-aromatization processes affecting “C6+” compounds become important from 350 °C/72 h. Thus, a substantial part of the nitrogen occurring in the “C6+” fraction can be incorporated in the insoluble residue when the sample is submitted to relatively drastic thermal conditions. This nitrogen is probably incorporated into polyaromatic, insoluble structures with a relatively high thermal stability. No ammonia was detected in any of the “gas” fractions generated under the different conditions tested. In contrast, ammonia was the main nitrogen species evolved during processing of oil shale samples from the Green River Formation.54 However, the bulk of the total nitrogen in these samples corresponds to ammonium ions “fixed” as such in clay minerals. In fact, the ammonia released during the above pyrolyses was shown to originate mainly from the mineral matrix and (54) Whelan, J. K.; Solomon, P. R.; Desphande, G. V.; Carangelo, R. M. Energy Fuels 1988, 2, 65-73.
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only a very low percentage of the organic nitrogen was released as NH3. A weak production of NH3 was previously observed, from coals of various ranks23 but, also in that case, NH3 probably originated from the mineral matrix. Indeed, a complete lack of ammonia formation was noted upon pyrolysis of coals with low clay contents and hence with low levels of “fixed” ammonium in the matrix.55 The lack of a significant production of ammonia from the NN8 kerogen seems therefore a common feature of fossil organic materials. However, a recent study, concerned with closed pyrolyses of anthracites under drastic conditions, 600 °C for 15 days,56 suggests that ammonia formation might be an important pathway for the release of organic nitrogen in the very last steps of coalification. Ammonia can be degraded into molecular nitrogen and hydrogen during pyrolyses but this degradation only takes place at high temperatures. In fact, previous control experiments24 confirmed that no significant NH3 degradation occurs under the closed pyrolysis conditions used in the present study. Accordingly, the N2 recovered during these experiments in sealed gold tubes can be considered as a direct pyrolysis product. If this N2 was derived from secondary degradation of NH3, this would both imply that substantial amounts of NH3 were produced and then totally transformed into N2, which is unlikely as discused above. N2 is the only nitrogen-containing gas generated during the pyrolysis of the kerogen (Table 4). This N2 production remains negligible up to 350 °C/1 h but, therafter, it shows a substantial increase. A maximum production of N2, corresponding to ca. 12% of the total nitrogen of the initial kerogen, is thus obtained for the 375 °C/72 h experiment. A much lower maximum of only 2.4% was observed, under the same conditions, from the type I Go¨ynu¨k kerogen.24 Moreover, in the case of this type I kerogen, nitrogen loss from the “C6+” fraction was not associated with a markedly higher production of nitrogen-containing gases and this lost nitrogen was mostly incorporated in the insoluble residue. In contrast, as shown by variations in distribution between the 350 °C/72 h and 375 °C/72 h experiments, a large part of the nitrogen lost by the soluble components is transformed into molecular nitrogen for NN8 kerogen. Another type II kerogen, from the Messel Oil Shale, was recently studied by closed isothermal pyrolyses.8 The latter study showed that the bulk of the nitrogen in this kerogen is also retained in the insoluble residue following pyrolysis at 365 °C for 3 days. However, these experiments were carried out under hydrous conditions and no direct measurements on the gaseous fractions were performed. Almost all of the previous studies on N2 release from fossil organic matter, upon a thermal stress, were concerned with nonisothermal open pyrolyses.6,7,14,17,20,23 Furthermore, these studies were carried out mainly with coals of different maturity levels, ranging from lignite to anthracite. Accordingly, precise comparisons with the present observations (isothermal closed pyrolyses on a type II kerogen) are difficult. In these previous studies, a part of the total nitrogen of the tested coals was shown to be extremely refractory (55) Bartle, K. D.; Perry, D. L.; Wallace, S. Fuel Proc. Technol. 1987, 15, 351-361. (56) Ader, M.; Boudou, J. P.; Javoy, M.; Goffe´, B.; Daniels, E. Org. Geochem. 1998, 29, 315-323.
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to thermal stress; in addition, it is well documented that the bulk of nitrogen in coals occurs in condensed aromatic (pyrrolic and pyridinic) units which exhibit a high thermal stability.46,57-60 The presence of such nitrogen forms therefore explains (i) the late formation of N2 from coals, via open pyrolysis at high temperatures, and (ii) the occurrence of N2-rich accumulations of natural gases such as those found in North Germany.6,7,14,17 In the case of the NN8 kerogen, the negligible production of N2 up to 350 °C/1 h points to the predominance of aromatic nitrogen-containing groups, like pyrrolic groups, in the unheated kerogen. Furthermore, as already stressed, polycondensation-aromatization processes become important from 350 °C/72 h and a substantial part of the “C6+” compounds, including nitrogen-containing ones, is thus transformed into insoluble material. As a result, polyaromatic nitrogencontaining units in insoluble condensation products are likely formed at stronger severity (375 °C/72 h). Such units should exhibit a high thermal stability and they probably contribute to the large percentage of the initial nitrogen still retained in the insoluble residue from the 375 °C/72 h experiment. Additional heating was therefore carried out so as to test the extent of the stability of the nitrogen retained in this residue. To this end, the 375 °C/72 h insoluble residue was further heated at 550 °C (maximum temperature at which the stainless steel cells can be used) for 24 h. No significant quantities of soluble “C6+” compounds were obtained following this pyrolysis at 550 °C, whereas an abundant “gas” fraction was formed. Indeed “C6+” compounds, if generated, are affected by extensive secondary reactions under such conditions36 and thus entirely transformed. GC analyses of the “gas” fraction (Table 4) indicated that no significant production of molecular nitrogen or of other nitrogen-containing gases occurred. As a result, the insoluble residue from additional pyrolysis under severe conditions retained ca. 80% of the total initial nitrogen It therefore appears that the aromatic nitrogen functions of the 375 °C/72 h pyrolysis residues exhibit an extremely high thermal stability. Such functions, not cleaved at 550 °C, should correspond to nitrogen-containing aromatic structures with a high degree of condensation. In contrast, an abundant production of N2 was previously observed, under the same conditions, from the 375 °C/72 h residue of the type I Go¨ynu¨k kerogen.24 In fact, the N2 thus released at 550 °C corresponded to ca. 50% of the total nitrogen contained in the unheated kerogen and the final insoluble residue retained ca. 25% of this total nitrogen, instead of ca. 80% for NN8. A higher thermal stability of the bulk of the nitrogen groups is thus observed for the type II kerogen when compared to the type I sample. It is well documented that type II kerogens are characterized by a larger contribution of aromatic units relative to type I kerogens.1 Such a difference likely accounts, at least in part, for the higher thermal stability of nitrogen in the NN8 kerogen. In fact, the larger level of aromatic units in the latter, (57) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100-104. (58) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 14501455. (59) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896-906. (60) Burnham, A. K.; Braun, R. L. Org. Geochem. 1990, 16, 27-39.
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before any heating, would promote a more efficient formation of polycondensed, nitrogen-containing, structure with a very high thermal stability upon pyrolysis. Regarding the other gases generated at 550 °C (Table 4), it appears that • Large additional productions of CH4 (derived both from primary cracking of the insoluble material and from secondary reactions) and, to a lesser extent, of CO2 and molecular hydrogen are obtained; • No C2-C5 hydrocarbons are observed, due to extensive secondary cracking into CH4; • A relatively large amount of H2S is formed; however, it is known that pyrite decomposes under such conditions. In the case of the Go¨ynu¨k kerogen, the concentrate obtained after HF/HCl treatment did not contain significant amounts of pyrite and a negligible production of H2S was observed, from the 375 °C/72 h residue upon further pyrolysis at 550 °C.24 As discussed above, nitrogen-containing groups exhibit a higher thermal stability in NN8 kerogen than in Go¨ynu¨k kerogen and this is probably related to a larger content of aromatic structure in the former material. A similar difference in stability should also occur, for the same reason, for sulfur functions. Thus, the presence of a larger level of aromatic units in the unheated NN8 kerogen would also promote the formation, upon pyrolysis, of polycondensed, sulfur-containing structure with a high thermal stability. Accordingly, it can be considered that virtually all the H2S generated from NN8 at 550 °C originated from the pyrite abundantly present in the heated residue. Geochemical Implications. On the basis of previous studies concerned with type II kerogens,40,60,61 it can be considered that the end of catagenesis and the onset of metagenesis for such kerogens is simulated by closed pyrolyses at 350 °C/24 h. Accordingly, the catagenetic production of the different gases to be expected, from the NN8 kerogen, during natural evolution, corresponds to the amounts generated during the 350 °C/24 h experiment. The production of these gases during the first stages of metagenesis was calculated, by difference, from the values obtained at 375 °C/72 h and 350 °C/24 h. However, the metagenetic productions of CH4 thus calculated is overestimated due to secondary reactions. In fact, as already stressed, the secondary cracking of the “C6+” compounds into light hydrocarbons becomes important from 350 °C/72 h under the closed conditions used for NN8 pyrolysis. In contrast, during natural evolution, generated fluids are expelled from source rocks and expulsion rate is especially high for compounds with relatively low molecular weights. As a result secondary cracking is negligible for such compounds but can be significant, even under natural conditions, for heavy components such as resins and asphaltenes.40 The productions upon “high temperature metagenesis” were directly provided by the results of the pyrolysis at 550 °C. In that case the production of CH4 under natural conditions is not significantly overestimated since (i) the “C6+” compounds were removed, by extraction, from the 375 °C/72 h residue before further heating at 550 °C, and (ii) only negligible amounts of additional “C6+” compounds can be gener(61) Tegelaar, E. W.; Noble, R. A. Org. Geochem. 1994, 22, 543574.
Nitrogen in the Pyrolysis Products of a Type II Kerogen Table 7. Absolute Amounts (µmol/g of initial kerogen) for CO2, N2, CH4, and H2S Generated during the Different Stages of the Simulated Maturation of NN8 Kerogena maturation stage
CO2
N2
CH4
H2S
catagenesis metagenesis “high-temperature” metagenesisb
759 (31.0) 570 (23.3) 201 (8.2)
20 (2.6) 74 (9.6) e
424 1839c 988
42 (5.4) 434 (55.3)c 247d
a These productions were derived from the values reported in Table 4. The bracketed values correspond to the percentage of the total oxygen from the initial kerogen released as CO2, the percentage of the total nitrogen released as N2, and the percentage of the total sulfur released as H2S. b Further heating (550 °C/24 h) of the insoluble residue from the 375 °C/72 h pyrolysis; as already discussed, footnote a, Table 4, these values cannot be directly compared with the others in the table, but they provide information on the metagenetic productions under a relatively drastic thermal stress. c Overestimated productions due to secondary cracking reactions and thermal degradation of pyrite, respectively. d The bulk of this production originates from pyrite degradation. e : see footnote b, Table 4.
ated from this insoluble residue. The assignment of laboratory time/temperature conditions to natural evolution stages used for these calculations was based on previous studies from three different groups.40,60,61 The use of laboratory values for extrapolation to geological conditions is a general problem for simulation studies and there are of course uncertainties. However, some shift in the limit of the considered stages would not significantly alter the main conclusions of the study discussed below. The calculated productions of CO2, N2, CH4, and H2S during the different stages of NN8 maturation are reported in Table 7. The following points can be noted: •The amount of released CO2 decreases, as expected, from catagenesis to metagenesis. Nevertheless, rather unexpectedly, a large potential for CO2 production is still retained after catagenesis completion and substantial CO2 production still occurs even during “high temperature metagenesis”. CO2 production appears as a major pathway for oxygen elimination from the NN8 kerogen and at 375 °C/72 h approximately 55% of the total oxygen contained in the initial material has been eliminated through this way. •The catagenetic production of N2 is low. A substantial increase in N2 production is observed during the first stages of metagenesis. However, this production remains moderate and less than 10% of the total nitrogen from the kerogen is then released as N2. Moreover, no significant formation of molecular nitrogen is obtained during “high-temperature metagenesis”. Accordingly, the bulk of total nitrogen in the NN8 kerogen corresponds to functions with a very high thermal stability, which are probably formed during maturation via aromatization-condensation reactions. The type I kerogen from Go¨ynu¨k previously studied 24 showed a still lower catagenetic production of N2, corresponding to only 0.5% of total nitrogen. But, in contrast, it exhibited a large production of N2, accounting for approximately 50% of total nitrogen, upon “high-temperature metagenesis”. •The production of CH4 is substantial as early as the catagenetic stage. As already stressed, the high value calculated for the first stages of metagenesis is markedly overestimated due to secondary reactions. An abundant
Energy & Fuels, Vol. 14, No. 2, 2000 439
production of CH4 occurs, as expected, during “hightemperature metagenesis”. •Relatively small amounts of H2S are formed during catagenesis and such a feature probably reflects the low level of aliphatic sulfides in the NN8 kerogen. As already discussed, the production observed during the first steps of metagenesis is overestimated and the bulk of the H2S in the 550 °C/72 h experiment originates from pyrite degradation. A substantial part, above 20%, of the total sulfur of the kerogen remains located in functions with a high thermal stability after this additional heating, probably in condensed thiophenes. Information can be derived, from the results reported in Table 7, on the productions to be expected for various gases as a result of the thermal maturation of the NN8 kerogens upon burial. This includes a low production of N2 when compared to CH4 and CO2 during catagenesis, i.e., the phase of oil formation, with a N2/CH4 ratio (v/v) of about 1/20; a similar situation for the first steps of metagenesis (although precise N2/CH4 ratio cannot be determined due to some overestimation of CH4 production); a negligible production of N2 during “hightemperature metagenesis”, whereas large amounts of CH4 and CO2 are still formed. Accordingly, in the gas produced during maturation of the NN8 kerogen, at least up to high-severity metagenesis as simulated by the 550 °C/24 h experiment, CH4 should be much more abundant than N2. Conclusions This study permitted the establishment, for the first time to the best of our knowledge, complete nitrogen of mass balances for thermal degradation of a type II kerogen. The main results from these closed pyrolyses of the NN8 kerogen under various temperature/time conditions indicated that: •Moderate amounts of nitrogen occur in the “C6+” compounds. These soluble compounds undergo secondary cracking from 350 °C/72 h, thus affording light C1C5 hydrocarbons and additional insoluble material. As a result of these secondary reactions, some nitrogen groups from the “C6+” compounds are transformed into N2, whereas others are implicated in the condensation reactions yielding insoluble material. •N2 is the only nitrogen-containing gas generated under all the tested conditions. Catagenesis and the first steps of metagenesis are characterized by low N2 productions, corresponding to only approximately 3 and 10% of the total nitrogen from the kerogen, respectively. Furthermore, no significant release of N2 is observed upon “high-temperature metagenesis”. Around 80% of the initial nitrogen is thus retained in the insoluble material in stuctures that exhibit a high thermal stability. The latter probably correspond to heterocyclic polyaromatic units with a high degree of condensation. This lack of additional production of N2 at 550 °C makes a conspicuous difference relative to the type I kerogen previously studied. •Large productions of CO2 occur both during catagenesis and the first steps of metagenesis. Moreover, a substantial potential for CO2 release is still observed upon “high-temperature metagenesis”. •H2S is the only sulfur-containing gas produced from the kerogen. The relatively small amount generated
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during catagenesis is consistent with the location of most of the sulfur, in the unheated kerogen, in aromatic groups. Substantial amounts of sulfur-containing “C6+” compounds are formed during these closed pyrolyses. A substantial portion of the total sulfur (>20%) is still retained in the insoluble residue after severe heating. This sulfur with a high thermal stability is probably located in polyaromatic thiophenic structures. Taken together, these observations derived from isothermal closed pyrolysis indicate that (i) the natural maturation of type II kerogens such as NN8 should be characterized by a low production of N2 at least up to “severe” metagenesis simulated by the 550 °C/24 h
Behar et al.
experiment, and (ii) this production should be largely exceeded by the formation of CH4. Further studies are in progress in order to characterize, at a molecular level, the nitrogen-containing compounds occurring in the soluble pyrolysis fractions. Acknowledgment. The authors thank T. Lesage for technical assistance. We also acknowledge Drs. J. Connan (Elf Aquitaine), J. L. Oudin (Total), and B. Beaudoin (Ecole des Mines de Paris) for financial support to B.G. EF990157G
